The technology required to scan documents has existed for approximately twenty years. Means to provide full-color output has also existed in the form of providing three separate color-filtered detector arrays which deliver three color-coded outputs. Historically, linear image sensor arrays, in the form of charge-coupled devices (CCD's) or self-scanned photo-diode arrays (or MOS arrays), have been used to accomplish the scanning. The early prior art scanning devices required at least three essential elements to capture the image of a grey-scale subject document: (1) a light source to illuminate the document, (2) an image sensing means, and (3) a lens system to focus the image of the document on the image sensor. The prior art provision of full color output, as opposed to a grey-scale output, required a fourth essential element or treatment, that of separating the image into three separate color-filtered images, each image impinging on a separate detector array, and thus essentially tripling the complexity of the device. The background treatment below applies generally to gray-level imaging; the addition of color imaging requires separate processing means of one or another form in addition to the basic scanning and imaging.
A typical configuration for a scanning device (a lens reduction image sensor system) is illustrated in FIG. 1. An original document 1 is illuminated by a light source 2. Since a CCD image sensor 3 is typically approximately one inch long, an optical lens 4 is required to reduce the image of the text on the document 1 so that a full-width image can be received in the CCD image sensor 3.
In addition, to obtain the necessary reduction, an optical distance of 10 to 30 cm is required between the CCD image sensor 3 and the document 1. This optical separation distance necessitates a rather bulky assembly for the overall scanning device, and for this reason, some prior art devices use sophisticated (hence expensive and difficult to manufacture) folded optical schemes to reduce the total physical size of the assembly.
An improvement on the system shown in FIG. 1 is shown in FIG. 2, which depicts a contact image sensor (CIS) system. In this device the reducing optical system is replaced with a full-width rod-lens system 5. This system allows one-to-one scanning of the document because the rod lens 5 and a hybrid image sensor chip 6 are of the same width as (or greater width than) the document to be scanned. This arrangement allows the distance between the image sensor and the document being scanned to be reduced to approximately 2 cm.
A cross section of such an improved prior art imaging system utilizing a hybrid image sensor chip 6 is shown in FIG. 3, which depicts the arrangement of the components within a housing with a cover glass 7 to receive documents. FIG. 4 is a block diagram of such an imaging system, with FIG. 5 showing some detail of the construction of a prior art hybrid image sensor array 6. In this hybrid package, a plurality of individual sensor chips 61 are butted end-to-end on a single substrate. The number of individual sensor chips chosen is dependent upon the desired width of scanning. The hybrid sensor array 6 also contains signal processing means to serially activate the individual chips and to process the outputs.
A block diagram illustrating the function of a typical prior art individual sensor chip 61 is shown in FIG. 6, with detail of the sensor elements shown in FIG. 7. (The structure and function of this chip is described in detail in U.S. Pat. No. 5,299,013, issued Mar. 39, 1994.) With reference to FIGS. 5-7, the individual sensor chip 61 comprises an array of photodetectors, an array of multiplexing switches, a dummy cell, a shift register, a built-in buffer, and a chip selector. In operation, the sensor chip 6 is triggered by a start pulse to the first-in-sequence individual sensor chip 61 which serially activates the photodetectors on the first individual sensor chip 61. After the signal from the last photodetector element of the first individual sensor chip 61 is read, an end-of-scan pulse is generated so that the next sensor chip in sequence is triggered.
The individual sensor chips 61 of most prior art devices utilize npn phototransistors as the sensing elements, as illustrated in the circuit diagram shown in FIG. 7. The npn phototransistors provide some gain for the detected light signal, and thus serve to increase the photosensitivity of the device. However, phototransistors are subject to several inherent shortcomings. The gain provided is not linear, being proportionately higher at low signal strengths and lower at higher signal strengths. This nonlinearity can be a significant problem in certain applications, such as those requiring uniform differentiation of gray scales, or the balancing of colors in color-scanning applications.
Further, the gain from the phototransistors is not uniform from transistor to transistor within a chip or within the array. The base of an npn phototransistor is formed by ion implantation. There is typically a .+-.5% non-uniformity across a wafer subjected to ion implantation. This non-uniformity results in a current gain variation of .+-.30% across the wafer. The non-uniformity of the gain yields a non-uniformity of the photoresponse of the same magnitude.
A still further drawback to a scanner with phototransistors is the problem of low-light-level thresholding due to emitter offset. Before the output current can flow through the phototransistor, the current must first overcome the emitter-junction forward-bias voltage. This threshold effect results in light signals of low intensity not being sensed by the phototransistor. This problem is most significant in color scanning applications.
Another shortcoming in sensor chips utilizing npn phototransistors is a reduction in sensitivity for high-density arrays. A phototransistor's sensitivity is proportional to its sensing area. As array density increases, the sensing area of an individual phototransistor is decreased drastically, and thus the array sensitivity is likewise decreased.
A final shortcoming in prior art devices is that they require the use of the BICMOS process for manufacturing due to the utilization of the phototransistor sensing elements. This process is more complicated than the standard CMOS process.
A substantial improvement in contact image scanners over those using phototransistors is described in co-pending patent application Ser. No. 08/532,926, filed Sep. 22, 1995 by Hsin-Fu Tseng and Weng-Lyang Wang, titled "CONTACT IMAGE SENSOR UTILIZING VOLTAGE PICKOFF". A further improvement is described in co-pending patent application Ser. No. 08/595,330, filed on Feb. 1, 1996 by Hsin-Fu Tseng and Weng-Lyang Wang, titled "CONTACT IMAGE SENSOR (CIS) USING VOLTAGE PICK-OFF AND CORRELATED DOUBLE SAMPLING (CDS). Those applications are hereby incorporated by reference in their entirety.
In brief, the devices described in the above referenced co-pending applications comprise a plurality of sensing elements, control and drive clocks, digital scanning shift registers, and various signal-enhancing processing means so that noise or offset in the signal can be cancelled by differentially combining signal plus background reference with background reference only. One example is illustrated by the block diagram of FIG. 8, wherein the signals from dummy sensors d.sub.1, d.sub.2, . . . d.sub.n, are individually subtracted from active signals from sensors S.sub.1, S.sub.2, . . . S.sub.n, to give an output largely free from noise and offset.
One major advantage of those co-pending disclosed improvements is that the scanners detect voltage levels proportional to the reflected light, as opposed to current generated in detector elements. These stored voltage signals may then be processed to reduce noise and offset by means such as correlated double sampling (CDS), as illustrated by the block diagram of FIG. 9 and by the single-pixel circuit detail of FIG. 10.
Another advantage of the co-pending disclosed improvements is that they eliminate the use of phototransistors in the CIS sensor chip, and thus permit standard CMOS processing. A still further advantage of those co-pending disclosed improvements is that the sensitivity of the sensing elements is independent of detector size, and as a result, very high density CIS sensor chip arrays with very high sensitivity and with noise-reduction processing can be realized.